The invention relates to multiple channel interferometers for use for example, as dilatometers as needed for advanced measurement capability in the field of low-expansion structural hardware.
In the prior art, the first multichannel interferometer, known to applicants, was a modified Twyman-Green interferometer using a single laser source and wavefront division to develop seven measurement beams. The fringe counting rate was 4 KHz. The major problems with this interferometer were dropouts due to low fringe rate capability, channel to channel drift because of a large diameter beam splitter. and laser noise resulting from optical feedback thereto. The above interferometer is described in "Multichannel Interferometer For Metrology" C. R. Pond, M. H. Horman, and P. D. Texeira, Applied Optics, Vol. 10, No. 9, September 1971.
An improvement of the above interferometer is disclosed in U.S. Pat. No. 4,105,336. The interferometer described in the patent was used successfully to test several graphite-epoxy structural elements with respect to expansion. As the testing became more demanding it became apparent that the fringe bandwidth was indequate. Vibration of the test articles sometimes produced fringe rates in excess of the 4 KHz fringe bandwidth. In attempts to overcome the foregoing problems the fringe rate was changed from 4 KHz to 100 KHz, and in order to partially compensate for the bandwidth increase, the retroreflector diameter was increased from 3 mm to 10 mm, producing an order to magnitude greater signal. This high fringe rate was found to be beyond the capability of the piezoelectrically driven spatial filter, beam combiner and modulator 71 disclosed in U.S. Pat. No. 4,105,336. The present invention was conceived and developed to overcome the foregoing problems.
Other prior art known to the applicants that may be pertinent is U.S. Pat. No. 3,780,217 in which acousto-optic frequency shifting devices are employed.